Devices for dimming lights in alternating current power distribution systems include variable transformers, rheostats, silicon controlled rectifiers and triacs. Triacs are especially useful for dimming lighting loads by remote control, by touch input devices and by the use of low voltage switches.
A triac is a semiconductor device of the thyristor family which shows inherent ON-OFF behavior, as opposed to a gradual change in conduction. Unlike a silicon controlled rectifier, a triac can carry current: in two opposite directions. Triacs are three terminal devices having two main terminals and a gate terminal. Triacs can be gated into the ON state and can carry current in either direction. While ON there is a very low resistance path from one main terminal to the other, with the direction of current flow depending on the polarity of the applied voltage. However, when a triac is turned OFF, no current can flow between the main terminals in either direction.
Since a triac can carry current in either direction, it is not limited to conduction only during 180 degrees per cycle, but can conduct for up to the full 360 degrees per cycle. A triac is used to control the average current flow to a load, and the average current delivered to a load can be varied by varying the amount of time during each cycle in which the triac is in the ON state.
A triac is triggered to the ON state by current through the gate. This gate current required to trigger the triac flows in opposite directions depending upon the relative polarity on the main terminals of the triac. A triac does not require continuous gate current once it has been fired. The triac will remain in the ON state until the main terminal polarity changes or until the main terminal current drops below the holding current. Holding current is defined as the minimum current between the main terminals of a triac below which the triac will be turned OFF. The average current through the triac can be reduced by delaying the time, after zero crossing for each half cycle, at which the triggering current is applied. The portion of the half cycle prior to application of the triggering current, during which current does not flow across the main terminals is referred to as the firing delay angle.
The use of a triac as a latching switch between a 60 hertz alternating voltage source and a load, such as an incandescent light, is illustrated in U.S. Pat. No. 4,396,869. A control voltage is applied to the triac in response to two counters and a level-setting voltage applied to the triac control circuit. The variation in the voltage applied to an incandescent light can be applied both up and down in response to the time in which the same control motion is applied.
There are a number of ways in which the phase angle of the triac gate can be changed to control the average power applied to the load. The use of a microcomputer to determine the phase angle for establishing the intensity of a load controlled by a triac is shown in U.S. Pat. No. 4,359,670.
Another means of controlling the average current delivered to a load is to use a specially designed microchip to regulate the application of a gate current LSI Computer Systems Inc. of Melville, N. Y. manufactures an integrated circuit which can be configured to respond to a single switch input to vary the phase angle or the firing angle delay. One such integrated circuit is commercially available as LSI Part Number LS7232. Another integrated circuit dimmer chip is available from LSI with the part designation SHD20051A. This latter chip can be used with a switch having separate up and down low voltage switch inputs and a triac in a dimming block which also includes means for using the line voltage to generate the low voltage inputs to the dimmer chip. This device, however, cannot be remotely controlled from a system controller such as those used in home automation systems or in programmable lighting systems. One such system controller is disclosed in U.S. Pat. No. 5,218,552. That system controller can be used to control a smart dimmer block attached to a fixture to be dimmed in the following manner. When the dimmer block is commanded to go to a selected dim percentage, system controller software issues two momentary ON commands followed by maintained OFF or down commands until the dimmer is at the desired percentage. However, it is further desirable that this dimmer block also be controlled independently of the system controller, for example directly from a low voltage switch which can operate even when the system controller is not active. The system controller can therefore not know the exact level of the fixture attached to the block. Therefore the system controller first causes the intensity of the light fixture to increase to full ON before ramping down to a desired intensity level. Such action is objectionable because users wish to directly change the intensity from the preexisting level to the intended level without going to full bright.
The triacs employed in the prior art present one other disadvantage. With current triggering or gate control circuits, triacs cannot be reliably used to control inductive loads. Therefore the dimmer blocks using conventional triggering circuits cannot be used to vary the speed of small motors, such as fan motors, nor can they be used to dim low voltage lights which employ step down transformers to change the voltage from line level, such as 120 Volts AC normally used in the United States, to the 12 Volts AC which is normally used with high intensity halogen low voltage lighting.